To reduce the size of microstructured devices such as semiconductor circuits (e.g. integrated, analogue, digital or memory circuits, thin-film magnetic heads) with the technique of optical lithography, it is desirable to improve the optical resolution limit of optical microlithographic projection exposure systems. Due to diffraction, the resolution limit in a first order approximation is inversely proportional to the numerical aperture of the projection lens of the microlithographic projection exposure system, with which structures are projected from a mask onto a substrate by a projection beam to form the microstructured devices there, e.g., by exposure of a light sensitive resist (which covers the substrate) with at least parts of the projection beam. For this, one focus is to increase the numerical aperture of the projection lens. Another focus is to reduce the used wavelength for the projection process, since the optical resolution limit is also proportional to this wavelength. For this reason, the historical development of optical lithography systems has been such that the wavelength of the light used in the projection process has been reduced from visible light to ultraviolet light to Deep Ultra Violet light (DUV light, such as 193 nm light, is produced, for example, by an advanced ArF excimer laser), which now is broadly used in mass production of semiconductor circuits. Today, mass production of high integrated circuits is usually done using microlithographic projection exposure systems with a projection light having a wavelength of 193 nm, and with the numerical aperture NA of the projection system, which project structures from a mask (or structured object) onto a substrate of much more than 1.0, sometimes even more than 1.3. In general, such high numerical apertures can only be achieved by the use of immersion systems whose principles are already described, for example, in DD 221563 A1 or in US 2006092533 A1.
For further reduction of the size of the microstructured devices, a further reduction of the wavelength of the projection light is desirable. Because in the deep ultraviolet wavelength range many optical materials become opaque due to molecular or atomic excitation, there are generally no suitable materials for optical lenses for wavelength below about 157 nm. Using even shorter wavelengths for the projection light, the projection lenses can only work with reflective optical elements such as mirrors or with diffractive optical elements. During the last years, enormous efforts have been made to develop optical microlithographic projection exposure systems, which work in the wavelength regime with wavelength less than 50 nm for the projection process. Systems working with a projection wavelength between 10 nm and 14 nm are described e.g. in EP 1533832 A1 or in US 20040179192 A1. Depending on the light sources which are available for the projection light of such short wavelengths, the wavelengths for the projection light may be even 5 nm or less. At wavelengths of 50 nm or less, the projection lenses of the optical microlithographic projection systems include only reflective optical elements such as mirrors and/or diffractive structures such as reflective diffractive structures. Projection systems using such a short projection wavelength in this extreme ultraviolet regime are known as EUV (Extreme Ultra Violet) lithographic projection exposure systems.
A simplified EUV lithographic projection exposure system 100 is schematically shown in FIG. 1. The system includes an EUV light source 1, producing EUV light with a significant energy density in the extreme ultraviolet or EUV spectral region, especially in the wavelength range less than 50 nm, preferably in a range between 5 nm and 15 nm. As EUV light sources discharged-produced or laser-produced plasma light sources are used, making use of e.g. xenon, tin or lithium plasma which generates the extreme ultraviolet light. Such sources irradiate unpolarized light under about 4 π solid angle. Other sources generate a spatially more directed and a more polarized beam of extreme ultraviolet light such as e.g. synchrotron radiation sources. Depending on the EUV light source 1, especially if an EUV plasma light source is used, a collector mirror 2 may be used to collect the EUV light of the light source 1 to increase the energy density or irradiance of the EUV radiation and form an illumination beam 3. The illumination beam 3 illuminates via an illumination system 10 a structured object M. The structured object M is e.g. a reflective mask, including reflective and non-reflective or at least minor reflective regions to form at least one structure on it. Alternatively or in addition, the structured object includes or consists of a plurality of mirrors which are arranged about side by side in at least one dimension to form a mirror arrangement such as a mirror array. Advantageously the mirrors of the mirror array are adjustable about at least one axis to adjust the incidence angle of the illumination beam 3 which incidence on the respective mirror.
It shall be understood that the terms reflective, minor reflective and non-reflective relates to the reflectivity regarding the EUV light of the illumination beam 3. Due to the very short wavelength of the EUV light, the reflective surfaces are usually coated if the angle of incidence for the EUV light is less than about 45°. The coatings preferably include a multilayer of predetermined layer materials with predetermined layer thicknesses. Such mirrors are usually used for incidence angles less or far less than 45° down to about 0°. For such mirrors a reflectivity of more than 60% can be achieved due to a constructive interference of the reflected EUV light which is partially reflected at the various material boundaries of the individual layers of the multilayer. A further advantage of such multilayer-coated reflective mirrors or surfaces is their property to work in addition as a spectral filter, to make e.g. an illumination and/or projection beam of the EUV lithographic projection system more monochromatic. Such coated mirrors are sometimes also designated as normal incidence mirrors in an EUV lithographic projection exposure system.
For larger incidence angles than about 45°, especially for much larger incidence angles such as angles of about 70° and even more, it is sufficient if the reflective surface includes a metal or a metal layer such as Ruthenium, or if the reflective surface consists of a metal or a metal layer, including e.g. Ruthenium. At such high incidence angles, the reflectivity can be increased up to 60% and more without the necessity of a multilayer as mentioned above. As a general rule the reflectivity increases with increasing angle of incidence. Such mirrors are also designated as grazing incidence mirrors. EUV lithographic projection exposure systems often use plasma light sources. In this case, the collector mirror 2 can be a grazing incidence mirror as described e.g. in WO 2002/065482 A2 or US 2004/0130809 A1.
The structured object M reflects parts of the illumination beam 3 into a light path which forms a projection beam 4. The projection beam 4 carries the information of the structure of the structured object M and impinges on a projection lens 20 such that at least two diffraction orders of the structure or structures of the structured object M pass the projection lens 20 and form a kind of an image of the structure or structures of the structured object M on a substrate W. The substrate W, e.g. a wafer, including a semiconductor material such as silicon, is arranged on a substrate stage WS which is also called wafer stage.
In addition to the information about the structure of the structured object M, the projection beam also carries information about the illumination condition of how the structured object M is illuminated regarding angular, polarisation and intensity (or radiation power per unit area) in an object point OP of the structured object M, and of how these parameters are distribution over the illuminated surface of the structured object M. The kind of illumination is expressed by the term “setting”. This means a predefined angular and/or polarisation and/or intensity distribution with which an object point OP is illuminated on the structured object M, and how these distributions depend on the spatial position on the structured object M. The setting also influences the optical resolution of the projection process, which is done by the projection lens 20. In general the optical resolution can be increased if the setting is adapted to the shape of the structure on the structured object M. Advanced illumination techniques which use adapted settings for the illumination of a structured object are described e.g. in “Resolution Enhancement Techniques in Optical Lithography” by Wong, Alfred Kwok-Kit; ISBN 0-8194-3995-9”. The kind of illumination (the setting) can be adjusted with the illumination system 10, which includes a plurality of mirrors 12, 13, 14, 15, 16.
Without loss of generality, the projection lens 20 schematically shows four mirrors 21, 22, 23 and 24 as reflective optical elements to form a kind of an image of the structure of the structured object M on the wafer W. Such EUV projection lenses 20 typically include 4 to 8 mirror. These mirrors are made with very high precision regarding surface figure (or regarding their geometrical form) and surface roughness. Each deviation regarding the desired specification results in a degradation of the image quality on the substrate or wafer W. Usually the specification is such that e.g. the deviation from the surface figure is less than a tenth of the used projection wavelength. Depending on the used wavelength the surface figures of the mirrors 21, 22, 23 and 24 are made with a precision of even better than 1 nm, for some mirrors the desired precision is even a factor of 5 to 20 higher, and up to precision ranges of much smaller than one atom layer or better than 0.1 nm. It is desirable to keep this very high precision regarding the surface shape (surface figure or geometrical form) over a mirror dimension of more than 10 cm. Modern EUV projections lenses include mirrors of a diameter of 30 cm or even more with such a high precision regarding the surface figure. This very high mechanical precision is desired to form an image point IP on the substrate W from an illuminated object point OP on the structured object M by illuminating the object point OP with a well configured illumination beam according to a predetermined setting. Further, to project the such illuminated object point OP with the projection lens 20 with at least parts of the projection beam 4 onto the substrate W, the projection beam 4 is generated by the illumination beam 3 and the diffracting properties of the structured object M. One desirable condition to form an image on the substrate W is that the diffracted wave fronts, which are coming from an object point OP on the structured object M, interfere in the image point IP on the substrate or wafer W. To get a good image quality the interfering wave fronts desirably have a relative phase shift of far less than one wavelength of the projection beam light. Due to the various illumination settings, of how the structured object M can be illuminated by the illumination beam 3, the light path of the light passing one object point OP on the structured object M can vary within the projection lens 20 in such a way that light bundles of the projection beam 4 are reflected by the mirrors 21, 22, 23, 24 of the projection lens 20 at different surface areas with different sizes. This variation depends on the illumination settings and the position of the mirrors 21, 22, 23, 24 within the projection lens 20. To make sure that the image quality is achieved under all illumination settings it is desirable that the above-mentioned surface figure is achieved with the mentioned high mechanical precision.
Apart from the high mechanical precision of the surface figure of the mirrors 21, 22, 23, 24 in the projection lens 20, also the position and orientation of these mirrors 21, 22, 23, 24 relative to each other, relative to the structured object M and relative to the substrate W are in the same range of accuracy. This means that position and orientation of these objects (mirrors 21, 22, 23, 24, structured object M and substrate W) are adjusted also in the nanometer range or even below.
In addition, a metrology is desirable to allow the manufacturing of such precise mirror surfaces, the assembling of the projection lens of the EUV lithographic projection system, the integration of the assembled projection lens into the projection system, and to allow any in-situ monitoring and control of the system during the operation of the system.
To correct position and orientation of at least one mirror 21, 22, 23, 24 of the projection lens 20 this mirror 21, 22, 23, 24 can be actuated in up to 6 degrees of freedom. This means that the position coordinates of the mirror 21, 22, 23, 24 can be adjusted in space.
The mirror position is usually described with coordinates of a first coordinate system such as a Cartesian coordinate system which is used as a reference system. Such a reference system is given in FIG. 1 as a xyz-coordinate system in which the y-coordinate is perpendicular to the drawing plane, directing from the top to the bottom. If the mirror 21, 22, 23, 24 behaves like an ideal rigid body the position of the mirror is definitely defined, e.g. by giving the coordinates of a reference point of the mirror, such as e.g. its centre of gravity S in the reference xyz-coordinate system, express as R=(Rx, Ry, Rx), as shown in FIG. 2. FIG. 2 schematically shows a cube shaped mirror body MB in the reference xyz-coordinate system. Of course, in the case of an ideal rigid body the position of the mirror body MB is also definitely defined if one arbitrary other reference point of the mirror is chosen, such as e.g. a corner A of the mirror body MB.
The orientations of the mirror(s) 21, 22, 23, 24 of the projection lens are usually described by three angle coordinates and, describing a relative orientation of a second coordinate system, which is fixed relative to the mirror body MB (or in general the rigid body) and which is designated as x′y′z′-coordinate system. These angles, e.g. chosen as Eulerian angles, describe the relative position of the coordinate axes of second x′y′z′-coordinate system to the axes of the reference xyz-coordinate system as also shown in FIG. 2. As one possibility, as shown in FIG. 2, the origin of second x′y′z′-coordinate system is located in the centre of gravity S of the mirror body MB, and the axes of this second coordinate system are parallel to the respective edgings of the cube shaped mirror body MB. However, the origin of the fixed second coordinate system can be at an arbitrary location in space in a way that the second x′y′z′-coordinate system is fixed relative to the mirror body MB. Also, the orientation of the axes of second x′y′z′-coordinate system relative to the mirror body MB may be arbitrary, but have a fixed relation to the mirror body MB. As shown in FIG. 2 as an example, the centre of gravity S or the origin of the fixed second coordinate system x′y′z′ is at a position R=(Rx,Ry,Rz) in the reference xyz-coordinate system. Further, the orientation of the mirror body MB is defined by the angular coordinates and 11 coordinates Rx,Ry,Rz, and are independent from each other, and each of these coordinates represent one degree of freedom for a movement of the ideal rigid mirror body MB. Depending on any movement constraints, caused by the bearings of the mirror body MB, the body can be moved in space in up to three independent translations such that the position R=(Rx,Ry,Rz) of e.g. the centre of gravity is moved from a first point to a second point by translation. Additionally, also depending on the constraints, the mirror body MB can rotate in up to three independent rotation axes to align the mirror body MB in space from a start alignment to an end alignment. Such an end alignment is expressed e.g. by the Euleran angles and According to these angles the end alignment of second x′y′z′-coordinate system, and so the mirror body MB, is achieved by first rotating the mirror body MB by a rotation r1 of angle about a axis z″ (being identical with the axis z′ in the start alignment of the mirror body MB), carrying the y″ axis (being identical with the axis y′ in the start alignment of the mirror body MB) over into an axis y′″ (see FIG. 2). After this first rotation, a second rotation r2 about the axis y′ is done by the angle his second rotation r2 is carrying the z″ axis (being identical with the axis z′ in the start alignment of the mirror body MB) over into the z′ axis of the end alignment. After this second rotation, a third rotation r3 by an angle about the z′ axis in the end alignment is done, carrying the axis y′″ over into the end alignment y′. Of course, any of the mentioned angles can be zero, if the mirror body rotates about less than three axes. FIG. 2 shows also the projection of the y′-axis of the end alignment onto the x″y″-plane of the x″y″z″-coordinate system which is designated as y″″-axis.
However, in reality, the mirror body MB is not an ideal rigid body and so the shape of the body of the mirror itself may vary in time due to e.g. long-term effects, thermal effects or the influence of forces and torques which act on the mirror body MB. In this case the shape of the mirror body MB is described in the x′y′z′-coordinate system which is fixed relative to the body of the mirror at one point, e.g. the centre of gravity S. Such deformations are schematically shown in FIG. 3 in which the shown deformations Δ are mainly caused by a temperature gradient grad(T(x′,y′,z′)) within the mirror body MB. In FIG. 3 the same reference numerals are designating the same elements as in FIG. 2. Such deformations can bring the reflective surface of the mirror to the limit of tolerance for the surface figure or even out of this limit. This results in an unacceptable degradation of the image quality of the EUV-projection lens 20. In an EUV-projection lens 20 such thermal deformations are in the range of about one nanometer even if the mirror bodies MB are made of glass ceramic materials such as e.g. Zerodur® (a registered trademark of Schott AG), or ULE® (a registered trademark of Corning Inc.) material. ULE® is a titania silicate glass, which is a vitreous mixture of SiO2 and TiO2. Both materials have a very low linear thermal expansion coefficient in the range of ± some ppb/K (parts per billion per Kelvin) or even zero for a certain narrow temperature range. The small deformations of the mirror body MB shown in FIG. 3 mainly result form temperature gradients inside the mirror body which can be up to 10K due to the absorption of parts of the projection beam 4. Even such small deformations of the surface figure of about 1 nm can be above the limit of tolerance.
As mentioned, the shape of the mirror body MB and therefore the surface figure of a reflective surface of the mirror 21, 22, 23, 24 can vary in time due to material properties and/or forces and/or torques or moments which vary in time. Since the accuracy of the surface figure is desirably maintained during the operation of the EUV-lithographic projection exposure system in the nm-range and even better, also the mounting system of the mirror and any actuation system for holding and actuating the mirror body MB fulfil extreme properties regarding mechanical precision. For this reason, the mounting system is desirably constructed in a way that any unintentional or parasitic forces and moments acting onto the mirror body MB are avoided, or are reduced as much as possible.
The above-mentioned extreme mechanical precision regarding position and surface figure of an EUV-mirror are realized in an EUV lithographic projection exposure system. To achieve this mechanical precisions very elaborated machine designs are involved, taking into account all factors of potential mechanical disturbances, such as e.g. mechanical vibrations, thermal effects, air pressure and gravitational influences, material properties and lots more.
It is known to mount optical elements with at least one operative optical surface, such as mirrors, in such a way that the optical element is supported, levitated or positioned with a single mounting device. Often the mounting device is positioned on a rear side of the operative optical surface (e.g. a reflecting or diffracting surface) of the optical element, or is positioned on a side being opposed or approximately opposed to the operative optical surface.
U.S. Pat. No. 6,068,380 describes a mounting assembly for a vehicle mirror with low demands regarding surface figure. The mirror is supported by a projecting ball portion on the mirror side being opposed to the reflective side of the mirror. This ball portion supports the mirror on a mirror support.
U.S. Pat. No. 5,035,497 describes a mirror support, including several mirror supporting devices for supporting a mirror in a certain attitude. The mirror support is constructed such that gravity does not influence a predetermined shape of the mirror surface. The mirror supporting devices support the mirror in the centre of gravity of the part of the mirror being supported by the respective mirror supporting device.
EP 1 780 569 A1 discloses a support mechanism for a reflecting mirror of high accuracy shape such as for telescopes. The support mechanism includes a bipod-like structure, including two legs for supporting the mirror. A centre axis is associated to each of the two legs along their supporting direction, having an intersecting point at the position of the centre of gravity of the reflecting mirror.
In US 2005/0248860 A1 a mirror actuator interface is disclosed to actuate a mirror in its neutral plane in up to six degrees of freedom. Further, the interface is compliant in certain degrees of freedom to minimize parasitic forces and moments or torques which are subjected to the mirror due to the actuator interface itself.
In DE 199 33 248 A1 a mirror telescope includes a primary mirror which directs the incoming light to a secondary mirror. The primary mirror is supported by a mirror body which includes a tube shaped mounting projection. The mounting projection is centred relative to the optical axis of the primary mirror and is drawn-out on the backside of the primary mirror. A similar space telescope is described in “Thermoelastic analysis, model correlation and related problems explained on the basis of the SILEX telescope” by Schoppach Armin published in “Spacecraft Structures, Materials and Mechanical Engineering, Proceedings of the Conference held by ESA”, CNES and DARA in Noordwijk, 27-29 Mar. 1996 (Edited by W. R. Burke. ESA SP-386. Paris: European Space Agency (ESA), 1996, p.627; Bibliographic Code: 1996ESASP.386.6275)
An additional space telescope is described in “SIRTF Primary Mirror Design, Analysis, and Testing” by Saver Georg et al. published in “Proc. of SPIE Vol. 1340, Cryogenic Optical Systems and Instruments IV, ed. R. K. Mehring, G. R. Pruitt (November 1990)”. There, a mirror assembly of a primary mirror is described in which the rear side of the mirror is arc shaped. Additionally the rear side of the mirror includes one flat section perpendicular to the optical axis of the mirror. The mirror is mounted onto a mirror support such that the flat section of the mirror is pressed against a flat section of the mirror support.
In WO 2008/010821 A2 a scan mirror is disclosed. The mirror is pivotally supported on a shaft arranged on a mirror side opposite of the reflecting surface. The shaft is pivoted on a holder and supports the mirror along a pivot axis. The pivot axis extends along a centre region of the mirror in a distance from the reflecting surface.
In U.S. Pat. No. 7,073,915 B2 a mirror fixing method and a mirror together with a mirror fixing device is described in which deformation of the shape of the mirror surface is reduced. The mirror includes a base plate carrying the reflecting surface on one side. On a side opposite to the reflecting surface a projection such as a bearing boss is formed or attached to the base plate. The mirror is fixed with the bearing boss.
In U.S. Pat. No. 7,443,619 an optical element holding apparatus is disclosed which holds a mirror suitable for an EUV lithographic projection exposure apparatus such that the mirror may be deformed to reduce any errors.
FIG. 4 shows schematically a mirror mounting assembly 400 with a mirror 421 as used in an EUV-lithographic projection exposure system 100, as described in e.g. in WO 2005/026801 A2. In addition, the reference xyz-coordinate system is given for orientation. The mirror 421 includes a mirror body MB made e.g. of Zerodur® or ULE®, or including e.g. one of the materials Zerodur® or ULE®. The mirror 421 also includes a reflective surface 450, including e.g. a multilayer of predetermined layer materials with predetermined layer thicknesses to improve the reflectivity of the projection beam 4 (FIG. 1). The mirror body MB is mounted by a kinematic mount at three mounting or linking points 451, 452, 453. At each of these mounting points the mirror body MB is connected with a bipod structure 461, 462, 463. At least one of these bipod structures may include an actuation device. Preferably, the actuation devices are Lorentz actuators, since these actuators are force controlled in the sense that the actuation force in a first order of approximation is proportional to an electric current or a magnetic field which results from the electric current. The bipod structures 461, 462, 463 are connected to the mirror body MB at the three linking points 451, 452, 453 by linking elements 471, 472, 473. Preferably each bipod structure 461, 462, 463 includes two Lorentz actuators such that the mirror body MB can be actuated in up to 6 degrees of freedom in the x-, y-, z-coordinates and the three Eulerian angles. The actuators are constructed such that there is no mechanical contact to a support structure 480 which is fixed at a housing structure 481 of the projection lens 20. The housing structure is sometimes also called a projection optical box or POB.